This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
There are two methods of nailing slate: head nailing and shoulder nailing. In head nailing, which is the usual method employed, the nail holes are punched about two inches from the top, as at a, Fig. 89, and the tails of the next two courses, as b and c, lap over the nail holes. Should the first-covering slate b become broken, the nails are thus still protected from the weather by the lap of the second-covering slate. The objection to this method is the leverage exerted by the wind. In shoulder nailing, Fig. 90, holes are punched at a distance from the tail of the slate equal to a little more than the gauge plus the lap.
117. The tail ends of slates are sometimes cut on the corners so as to give, when laid, a semi-octagonal or triangular gauge, and when so treated are termed cut slates.
Cut slates tend to shed the water more rapidly than square-end slates, as their form acts as a guide, carrying the water to a point, which it readily leaves, thereby clearing the roof quickly; while, with square slate, the water lodges in the joints and accumulates on the lower edge and drips off so slowly that the joints are wet when the rest of the roof is dry. In winter, the alternate freezing and thawing is likely to loosen the slate and cause the edges to crumble. A roof covered with cut slate is, however, not so strong, as so much of the material is cut away.
118. Sorting and piling slate preparatory to laying is a most important detail of the roofer's work. Slates should be piled with their edges up, the pile in no case being more than 3 feet 6 inches in height; the ends of the tiers may be held up by laying a pile of slates on the flat, while the top of the pile should be covered with slate laid flat, with broken points, so as to keep out moisture. The slate should be sorted or selected by grades of thickness, the thin slate being piled first and the thicker ones next; or, if there are three grades, the medium thickness should be piled before the thickest.
The thick slate should be punched first, then the thinner ones, as the thick slate should be laid on the lower part of the roof and should be taken up first.
119. Machine-punched slate make a better job than hand-punched. In Fig. 01 is shown a section through a machine-punched slate; the hole is clean cut and gives the very best result.
In Fig. 92 is shown a hand-punched slate whose edge is ragged and flaked, the slate consequently being weakened. In Fig. 93 is shown another machine-punched slate, but with the nails driven too tight. The result is as indicated. The slate having sprung to its natural position, the under side is splintered and the value of the nailing is lost. In Fig. 94 is shown a machine-punched slate with the nail insufficiently driven, causing the nail head to be forced through the upper slate by pressure from above.
120. There are three methods of laying slate roofing: (1) on laths or battens; (2) on boards covered with felt; and, (3) a method called half slating, on either battens or boards.
With the first method, of lath or battens, the roof is prepared as for shingles laid on lath, as previously explained.
For the second method, where the slates are laid on boards, the best results are obtained by covering the roof with 1 1/4" X 6" matched boards. The objection to wider sheathing boards is that they are likely to shrink, curl on the edges, and lift some of the slates, thereby giving the roof a rough, uneven appearance. If wide boards are used, however, they must be well nailed at both edges. More care in this respect is needed when the sheathing is applied to curved roofs, or round towers, in which cases the sheathing must be perfectly solid and smooth. When the sheathing is not solid, the driving of a nail is almost sure to loosen the slate which was previously nailed, and a uniformly tight job is impossible.
The sheathing felt should be applied parallel to the eaves, if possible, but if the roof is steep it may be laid at right angles to them. In either case it must be lapped 2 inches and fastened to the roof with nails having large flat heads. The tilting fillet at the eaves, valleys, gable ends, etc., the saddles and cant boards at the back of chimneys, flashings, etc., as previously described, should all be well secured to the roof boards.
In commencing to slate, the first course is laid double, the lower, or "undereaves" course being laid with the back of the slate next the boards; the length of the slates in this course will be equal to the gauge plus the lap, as seen at a, Fig. 95, and should project over the eaves or gutter edge from 1 1/2 to 2 inches, as at b. All the courses must be laid with broken joints, as shown at c, up to the ridge; the last course, as at d, is known as a "finisher," and is put on to receive the ridge roll.
121. The third method, known as half slating, is generally used on cheap buildings, where great economy of material is required. If laid on battens, half-slating should be used only where a perfectly tight roof is not necessary; but if laid on boards covered with felt, it makes a good serviceable low-priced roof. The lower courses should be laid double, as in the other methods. The courses above this have a space between the edges of the slates equal to one-half the width of each slate. The whole surface is thus laid, up to the ridge, the last course, or finisher, and the ridge roll being completed as for the other roofs.
122. The ridge on a slate roof may be finished in various ways. A method used on cheap buildings has a metal saddle flashing, as at a, Fig. 96, with the slate finished over it, one slate lapping over the other 1/2 inch, as at b. The wings of the metal flashing are just equal to the length of the ridge slate c. The objection to this form is that the nails securing the ridge slates are driven through the metal flashing.
Another method consists in nailing to the roof boards a wooden ridge saddle or strip a, Fig. 97, on each side of the ridge. A saddle flashing b is then bent over this, with the wings extending 4 or 5 inches down the roof and under the last course of slates, which are laid up to the edge of the saddle a. Strips of metal c, 8 in. or 10 in. X 2 in., are nailed along the saddle at intervals of 1G inches, and a wing piece d is then laid on each side, lapping over the slate 2 or 3 inches, and is bent and shaped up to the top of the clip c, over which it is turned and locked, as shown at e. The whole is then bent and formed into a roll, as indicated by the dotted lines.
A third method, much the same in principle, is shown in Fig. 98. The metal saddle flashing over the wood saddle is omitted, and the slate is carried up to the wood saddle, on which is nailed a wood roll, as shown at a. A wing flashing b is placed each side, overlapping the slate 2 or 3 inches, and is bent up against the roll and lock seamed as at c, with its upper edge against the side of the roll; this flashing is then secured by nailing under the lock seam into the wood roll. A metal roll cap d with lock-seam edges, slipped over the wood roll, interlocking with the edges of the wings, completes the ridge. A cheaper roll is constructed as shown in Fig. 99, the roll a being kept in place by its natural spring.
Methods of capping with slate rolls and wings are seldom used on account of their expense, but when necessary should be constructed as shown at (a) and (b), Fig. 100.
At (a) the wing pieces a are cut from solid slate; they are screwed together with brass screws b, and the cap c is placed over them. At (b) the wing piece a has the roll cut on it and laps the wing b on the other side. Both ridges must be set in cement, without nailing to the roof.
A good rule to follow for proportioning the wings to the roll is: with a 2-inch roll use a 5 1/2-inch wing; with a 2 1/2-inch roll use a 6-inch wing; and with a 3-inch roll use a 7-inch wing.